The exhaust gas discharged from a vehicle using gasoline as fuel contains harmful components such as hydrocarbons (THC), carbon monoxide (CO), and nitrogen oxides (NOx). Hence, it is required to purify each of the harmful components so as to convert the hydrocarbons (THC) into water and carbon dioxide through oxidation, carbon monoxide (CO) into carbon dioxide through oxidation, and the nitrogen oxides (NOx) into nitrogen through reduction.
As the catalyst (hereinafter, referred to as the “exhaust gas purification catalyst”) for treating such exhaust gas, a three way catalyst (TWC) which is able to oxidize or reduce CO, THC, and NOx is used.
As such a three way catalyst, one that is obtained by supporting a precious metal on an oxide porous material having a high specific surface area, for example, an alumina porous material having a high specific surface area and supporting this on a substrate, for example, a monolithic substrate having a refractory ceramic or metallic honeycomb structure or on refractory particles is known.
Meanwhile, the exhaust gas discharged from a diesel engine contains sulfates derived from the sulfur component in the fuel and tar-like fine particulate materials (referred to as “PM”) due to incomplete combustion, and the like in addition to CO, THC, and NOx described above.
As a device for removing CO and THC contained in the exhaust gas discharged from a diesel engine, a diesel oxidation catalyst (referred to as “DOC”) is known.
As DOC, one that is obtained by coating a refractory inorganic porous material such as zeolite or Al2O3 on a porous filter substrate having a honeycomb structure is known.
A precious metal such as platinum (Pt), palladium (Pd), or rhodium (Rh) has been often used in both of the catalysts that are a catalyst for purifying the exhaust gas discharged from a gasoline engine and a catalyst for purifying the exhaust gas discharged from a diesel engine as a catalytically active component. Moreover, the bonding strength between these precious metals as a catalytically active component and the substrate is not so strong and the specific surface area of the substrate itself is not also so great, and it is thus difficult to support the precious metal on the substrate in a sufficient supporting amount and a highly dispersed manner even when it is attempted to directly support the precious metal on the substrate. Hence, it is conducted to support a precious metal on a particulate catalyst carrier having a high specific surface area in order to support a sufficient amount of a catalytically active component on the surface of a substrate in a highly dispersed manner.
As this kind of carrier for exhaust gas purification catalyst (also referred to as the “catalyst carrier” or “carrier”), porous particles composed of refractory inorganic oxides such as silica, alumina, and titania compounds are known. Among them, a catalyst obtained by supporting a precious metal on an inorganic porous material such as alumina having a high specific surface area in a highly dispersed manner has been hitherto widely used from the viewpoint of excellent purification performance at a low temperature.
With regard to the catalyst carrier, for example, a heat resistant catalyst carrier which is composed substantially of alumina and has a coating layer of one or more kinds of oxides selected from SiO2, CaO, SrO, BaO, and La2O3 is disclosed in Patent Document 1 (JP 6-218282 A (Kurosaki Fire-Brick Manufacturing Company Limited)).
A composite oxide carrier which contains TiO2—Al2O3 as a main component and Si and is characterized in that Si constitutes a composite oxide with at least either of TiO2 or Al2O3 is disclosed in Patent Document 2 (JP 2000-157865 A (TOYOTA MOTOR CORPORATION)).
A catalyst carrier for supporting a precious metal, which is composed of a composite oxide of an electron accepting element with other elements and in which the electronic accepting element is selected from the group consisting of lanthanum, neodymium, yttrium, magnesium, and any combination thereof, the other elements are selected from the group consisting of silicon, aluminum, zirconium, titanium, and any combination thereof, and the molar ratio of the electron accepting element to the sum of the electron accepting element and the other elements is 0.3 or more is disclosed in Patent Document 3 (JP 2007-144393 A (TOYOTA MOTOR CORPORATION)).
Spherical beads which has an average diameter in a range of from 10 to 120 μm, a BET surface area in a range of from 400 to 800 m2/g, and a pore volume in a range of from 0.3 to 3.0 cm3/g and contains a metal and/or a metalloid oxide selected from the group consisting of SiO2 or Al2O3, TiO2, MgO, and any mixture thereof is disclosed in Patent Document 4 (JP 2012-520236 W (BASF SE)) as the beads which can be utilized as a catalyst carrier.
A catalyst carrier for exhaust gas purification that is characterized by containing a phosphate represented by a general formula MPO4 (in the formula, M is Y, La, or Al) or zirconium phosphate represented by a formula ZrP2O7 is disclosed in Patent Document 5 (JP 2013-252465 A (MITSUI MINING & SMELTING CO., LTD.)) as a catalyst carrier for exhaust gas purification which can significantly improve the performance as compared to Rh-supporting zirconia while suppressing a decrease in NOx purification activity in a lean region in which the excess air ratio λ is greater than 1.